The present invention relates to a liquid-crystal panel for use in a liquid-crystal display apparatus, an optical shutter, or the like, to a method of manufacturing the same, and to a liquid-crystal display apparatus using the liquid-crystal panel.
Liquid-crystal panels, which are small in size, light in weight, and operable with low power voltage, have a wide range of applications as means for displaying a variety of visually recognizable patterns. For example, such appliances as wrist watches, electronic tabletop calculators, personal computers, and personal word processors often use liquid-crystal panels therein for their further miniaturization and weight reduction. However, since a liquid-crystal panel of TN (twisted nematic) type or STN (supertwisted nematic) type currently used requires a polarizing plate, its light transmittance and long-term light resistance are unsatisfactory. By contrast, since a panel containing a polymer dispersed liquid crystal (PDLC) does not require a polarizing plate, it is free from the above-mentioned drawbacks.
FIGS. 5(a) and 5(b) are cross-sectional views showing the structure of a conventional liquid-crystal panel of polymer dispersed type. FIG. 5(a) shows the state without a voltage applied, while FIG. 5(b) shows the state with the voltage applied. As shown in FIGS. 5(a) and 5(b), the liquid-crystal panel is constituted by: a pair of glass substrates 1a and 1b having electrodes 2a and 2b, respectively, which are opposed to each other; and a mixture of a liquid crystal 4 and a photo-curing polymer 5 which is filled into the space between the both substrates 1a and 1b. It is also possible to use a thermosetting polymer instead of the photo-curing polymer 5.
In a liquid-crystal panel of polymer dispersed type as described above, the liquid-crystal 4 is dispersed and held in the photo-curing polymer 5 or the photo-curing polymer 5 exists in particles or in network in the liquid crystal 4. If a voltage is not applied to the space between the electrodes 2a and 2b, liquid-crystal molecules are randomly oriented so that a difference in refractive index is caused between the photo-curing polymer 5 and the liquid crystal 4, thereby scattering light (see FIG. 5(a)). On the other hand, if the voltage is applied to the space between the electrodes 2a and 2b, the liquid-crystal molecules are oriented in the direction of a resulting electric field so that there is a match in refractive index between the liquid crystal 4 and the photo-curing polymer 5, thereby transmitting light (see FIG. 5(b)). Thus, the liquid-crystal panel of polymer dispersed type utilizes the phenomena of light scattering and light transmission. Spacers dispersed between the substrates 1a and 1b in order to maintain a specified distance therebetween are composed of an insulating material in spherical particles. Specifically, spacers made of a transparent material such as SiO.sub.2, benzoguanamine resin, or melamine resin are used at present.
However, the above conventional liquid-crystal panel is disadvantageous in that, when a voltage is not applied to the liquid-crystal panel, i.e., in the black mode, light scattering becomes unsatisfactory in the state shown in FIG. 5(a), so that a sufficient contrast cannot be obtained. Even when the voltage is applied to the liquid-crystal panel, local variations are observed in light transmittance, resulting in uneven display of images.
In view of the foregoing, the present inventors have examined the phenomenon closely, which led to the following findings.
(1) The spacers composed of a transparent polymer material presents no problem if they are used in the liquid-crystal panel of TN type or STN type. However, if they are used in the liquid-crystal panel of polymer dispersed type which utilizes the phenomena of light scattering and light transmission, light scattering becomes unsatisfactory when a voltage is not applied, i.e., in the black mode, so that a sufficient contrast cannot be obtained. PA1 (2) The use of spacers composed of an opaque colored polymer material, instead of a transparent material, is also under consideration as disclosed in Japanese Laid-Open Patent Publication Nos. 63-157130, 1-144021, and 4-15623. Even with the spacers composed of a colored material, however, it is difficult to obtain a sufficient contrast. PA1 (3) When a phase separation is caused by irradiating the photo-curing polymer with light or by heating the thermosetting polymer, liquid-crystal droplets adjacent to the spacers become larger in size than those not adjacent to the spacers. As a result, the application of a voltage causes a difference in light transmittance between the regions adjacent to the spacers and the other regions, so that a uniform displaying property may not be obtained in a liquid-crystal display apparatus.